How can I make a text input mandatory from a drop down selection that is greater than zero?

Make txtBox mandatory when the select value is greater than 0. There will be an alert popup making the text input mandatory.

i.e.

<select>
<option selected="selected" value="0">Select</option>
<option value="1">One</option>
<option value="2">two</option>
<option value="3">three</option>
<option value="4">four</option>
</select>

<input type="text" name="txtBox">